Currently, a non-aqueous electrolyte secondary battery including a lithium ion secondary battery, which is used for a mobile device such as a mobile phone, is available as a commercial product. The non-aqueous electrolyte secondary battery generally has a constitution that a positive electrode having a positive electrode active substance or the like coated on a current collector and a negative electrode having a negative electrode active substance or the like coated on a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution or a non-aqueous electrolyte gel is maintained within a separator. According to absorption and desorption of ions such as lithium ions on an electrode active substance, charging and discharging reactions of a battery occur.
In recent years, it is desired to reduce the amount of carbon dioxide in order to cope with the global warming. As such, a non-aqueous electrolyte secondary battery having small environmental burden has been used not only for a mobile device or the like but also for a power source device of an electric vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), and a fuel cell vehicle.
As a positive electrode active substance used for a non-aqueous electrolyte secondary battery, a lithium manganese composite oxide (LiMn2O4) is known. LiMn2O4 has a spinel structure and, by having composition with λ-MnO2, it is known to function as a positive electrode material at 4 V level. Since the lithium manganese composite oxide with a spinel structure has a three dimensional host structure that is different from a layer structure of LiCoO2 or the like, most of the theoretical capacity can be used, and thus it is expected to have excellent cycle characteristics. Furthermore, compared to LiCoO2 or the like containing cobalt, that is, a rare metal, it also has an advantage of relatively low cost.
Meanwhile, the non-aqueous electrolyte secondary battery which is hopefully to be applied for an electrically driven vehicle is required to have, in addition to the high capacity as a basic requirement, many excellent characteristics such as high output characteristics. To deal with these demands, various techniques for improving battery characteristics by using two or more positive electrode active substances in combination have been suggested. For example, in JP 2011-18547 A, as a technique for enabling prevention of at least one of over discharge and over charge while having stable output characteristics, combined use of a first positive electrode active substance (LiMnPO4) which is responsible, as a main component of a positive electrode active substance, for performing two-phase coexistence type charge and discharge, and a second positive electrode active substance (LiFePO4) which is also responsible, as a minor component, for performing two-phase coexistence type charge and discharge is disclosed.
Herein, an electrically driven vehicle such as an electric vehicle has a function of displaying a remaining capacity of a secondary battery to be installed or a remaining cruising distance and providing the information to a driver. Thus, based on the displayed information such as a remaining battery capacity or a cruising distance, the driver can determine the possibility for continuous driving to a destination or the necessity for charging, and if necessary, he can take countermeasures such as driving according to a driving mode that puts great value on a cruising distance, heading to a charging stand, or the like.
When display of a remaining battery capacity is performed, the cell voltage value of a battery is determined by real time detection. Subsequently, a corresponding DOD value to the detected cell voltage value is determined in view of a discharge curve, which is obtained (in advance) by plotting cell voltage against DOD (depth of discharge [%]). Then, from the obtained result, the remaining battery capacity can be calculated as follows:Remaining battery capacity=(100−DOD)[%].